This invention relates in general to exposure control systems. In particular, it relates to an improved method and optical assembly for use in controlling scene radiation in a light detecting device of an exposure control system usable in an image recordable apparatus.
Automatic exposure control systems are well-known in the photographic arts. Typically, such systems include a photometer having a photoresponsive transducer and a lens element or system. The lens system receives scene radiation from a preselected area of the scene to be photographed and directs such scene radiation onto the photosensitive surface of the transducer. The transducer's output may be utilized to predict a photographic exposure condition or actually control the closing of an automatic shutter mechanism at an appropriate film exposure condition. The output signal from such photometers is directionally responsive to the influence of the scene radiation from various scene objects within its field of view. Because photometers are directionally responsive, highly accurate control of exposure is difficult to obtain. In this regard, most of the exposure control systems known in the prior art employ a single photocell. Single cell photometers, however, present certain drawbacks especially during flash and ambient modes. In this regard, single photocells tend to react to the average brightness of a field substantially corresponding to the camera's field of view. Unfortunately, the average field brightness often differs from the brightness of selected zones. For instance, central or lower zones of the field have been found to be more representative of the brightness of the subject. Thus, when the brightness of these peripheral or upper field zones contrast with the brightness of central or lower field zones, an incorrect exposure for the more important subject normally occupying the center or lower zones results. Consider, for example, horizontal landscapes illuminated by skylight or sunlight, it is desirable generally to have a photometer look generally downwardly so as to minimize the influence of the sky. When there is a portrait scene with an artificial light source, it is desirable to have a photometer look straight ahead. This is done so that it receives substantially all the light reflected from the central portion of the scene and thereby properly exposes for flesh tones.
To correct for the noted drawbacks several proposals have been put forward. One has been to alter the acceptance angle of the photometer so that it evaluates scene radiation from different scene zones or portions. This is typically done in photographic systems having distinct exposure modes, for example, systems having distinct flash and ambient modes. Examples of such acceptance angle altering devices are shown in the following U.S. Pat. Nos: 3,511,142; 3,232,192; and 4,173,400.
Another proposal is to use a multi-photocell array. Commonly-assigned U.S. Pat. Nos. 3,511,142, 3,529,523 and 4,285,583 disclose photometric devices using an array of photocells arranged so that under certain conditions, each photocell receives light from a preselected portion of the entire scene. In U.S. Pat. No. 3,529,523 there is disclosed means for selectively interposing a device relative to a plurality of photocells so that during the flash mode each photocell images the entire scene and during the ambient mode each photocell receives an equal portion of the entire scene.
Recently, cameras have been proposed which do not have distinct flash and ambient exposure modes. Such cameras are referred to generally as proportional fill-flash types. The exposure control systems used in such types operate to control firing of an electronic quench type strobe under a range of natural lighting intensities so that film exposure is due to a mixture of available natural light and strobe light. The strobe light is proportioned in accordance with the intensity of the natural available light. In such system, it has been determined that exposure is also enhanced significantly if the flash and ambient modes are spectrally filtered differently. More to the point, it has been determined that during the flash, the evaluation of infrared frequencies, which frequencies normally are blocked during ambient, improves exposure.
One known approach to further improve exposure in such cameras is disclosed in commonly-assigned U.S. Pat. No. 4,345,828 wherein the acceptance angle is shifted automatically during the exposure mode so as to accommodate for flash and ambient modes.
To further improve exposure in such types of cameras it has been also proposed to use a pair of photocells so that one is used during ambient and the other is used during a pulse of artificial illumination. In this regard, commonly assigned and copending U.S. Pat. application Ser. No. 108,381, filed Dec. 31, 1979, by Bruce K. Johnson, and entitled "Method and Apparatus Employing Dual Photocells For Removing Spectral Filter During Strobe Pulse", discloses the use of a pair of photocells each of which is responsive to different spectral frequencies during flash and ambient modes. In particular, there is provided an electronic switching arrangement for automatically switching between photocells so that one is spectrally filtered during the ambient portion of an exposure interval in a different manner than the other during flash. A problem arising from such arrangement is optical crosstalk. That is, undesired spectrally filtered scene radiation intended for one of the photocells travels to the other photocell.
It is known to use mechanical light controlling elements to block crosstalk. Practically speaking, however, use of mechanical light controlling elements presents several difficulties, especially in systems of the foregoing type. This is because space considerations dictate that such mechanical elements be exceedingly small. Thus, there are potential problems with attempting to accurately position them as well as the potential that such small elements might, when once in use, become dislodged so as to adversely effect their intended light controlling function.
While it has been proposed to use the concept of total internal reflection for redirecting scene radiation in photographic apparatus, such proposals have, for the most part, used relatively large prism structures. These provide for highly-folded light paths between the objective and the film plane. Examples of such an approach are disclosed in commonly-assigned U.S. Pat. Nos. 4,119,980 and 4,109,263.
None of the known approaches have, however, attempted to solve the problems of optical crosstalk associated with closely spaced photoresponsive regions in exposure control systems, let alone solve such problems in a simple, efficient and economical manner.